The present invention relates to a method of drilling a well such as a geothermal well and, more particularly, to a process for drilling a well by utilizing AE/MA method (Acoustic Emission/Microseismic Activity).
When a geothermal well, a petroleum well, a natural gas well or a spring well is drilled by known methods, a bit is rotated to drill the well. In this case, drilling fluid is used to constantly remove drilling chips from the bottom of the borehole or the vicinity of the pit, to convey the chips to the ground to clean the borehole, to cool the bit, a drill collar and drill strings, to smoothly rotate the bit and to form a thin and tough impermeable membrane on the wall of the borehole.
As shown in FIG. 1, in the case of generally drilling a well with drilling fluid, a bit 1 of the end of a drilling conduit 2 is rotated by a rotary machine 3 through the conduit 2 and the collar 4, and drilling fluid (mud) 5 is supplied through the conduit to the end of the bit 1, discharged externally from the end of the bit, and recovered on the ground through the outside of the conduit. The drilling chips in the drilling fluid are separated and removed from the recovered drilling fluid, additives for drilling fluids are then suitably added, and the drilling fluid is recirculated for use.
However, when drilling the well with drilling fluid, the used drilling fluid is frequently lost while drilling, and so-called lost circulation frequently occurs.
Causes of lost circulation generally are known to include an encounter with rough particularly permeable formations or previous cracks, hydrostatic pressure of the drilling fluid and/or surge pressure produced by the pumping pressure for circulation or drilling conduit as artificial pressure.
If such lost circulation occurs while drilling in this manner, it is a disadvantage that drilling fluid of a larger quantity than otherwise required must be supplied resulting in huge expenses for drilling.
Heretofore, known methods of detecting if lost circulation occurs while drilling include a method of detecting the position of the lost circulation by moving a thermometer down into a well to measure the temperatures of the drilling fluid and the formation, and detecting the position of the lost circulation from the temperature change, and a method of detecting the position of the lost circulation by moving a small-sized spinner unit down into a well and detecting the position from the rotating speed difference of the spinner.
The conventional methods of detecting the position of the lost circulation during drilling the well can be carried out only after the lost circulation occurs, and it has such a disadvantage that a considerable amount of drilling fluid is still lost in fact.